The only hardware-accelerated operations of the GDI are color filling, blitting, alpha-blending, and stretching (with or without alpha). Everything related to drawing shapes (polygons, curves,
If anything feels snappier in older OSes, it would be because before XP, Microsoft still supported old hardware blitters, such as what was used by DirectDraw. Support for hardware blitting was lost with the switch away from the Win9X architecture and embracing NT. I may be remembering wrong on this, but I believe XP was missing the faster blitting, but didn't support the same level of hardware-acceleration as newer versions of Windows, so in theory, it should have been the least snappy of all...
As to the % of applications using GDI, I'd make it more like >95%, and I'm being conservative. The only application I know for certain uses Direct2D or Direct3D for the interface drawing would be Visual Studio 2010 or newer. And XUL-based applications such as Firefox, where d2d/d3d is supported and available. As far as I know even Java applications which custom-draw the GUI end up doing so with GDI functions.
XP was complete bloat by 2001's standards. Windows 7 was made lighter than Vista, after people complained about Vista the same way people complained about XP.
The only reason XP doesn't feel bloated right now, is because it's 13 years old and 10 years obsolete -- since it can't do a lot of things OSes are expected to do (not by you obviously). The biggest mistake Microsoft made wasn't Vista's expectation of a decent computer, nor windows 8's oversimplification. It was letting people grow used to XP for way too long.
Both my desktop computer AND my laptop have one thing in common: neither of them is a tablet. And Windows 8 as is is ONLY oriented towards tablets.
A lightweight OS oriented in low power usage and touch-based controls, which just happens to still maintain some sort of classic experience because they couldn't be arsed to remove it the way they removed other perfectly functional features, is not the OS I want to use.
Windows 9 may or may not be good enough to get into my computers. We'll see.
The trick for me to get used to Office 2007/2010, was to forget where things used to be, and search them where they are supposed to be. Then everything clicked into place, and now I like the UI quite a lot.
Note that I don't mean the SAME UI works for everything. That's a mistake some people seem to make. Different kinds of software are best with different interfaces.